This invention relates to tonometry and, in particular, to a non-contact tonometer for determining the health of living tissue.
Tonometry broadly relates to the measurement of tension in living tissue and has special meaning in ophthalmology relating to the intraocular pressure and health of the eye. Pressure in the eye is not measured directly, but is typically inferred by measuring the eye's response to pressure exerted upon the cornea.
One of the most widely employed eye tests is an applanation test devised by H. Goldmann in which the cornea of the eye is flattened by a device having a plane contact surface. The cornea is depressed until the depression reaches a predetermined radius at which time the amount of force required to produce the depression is noted. This force is related to intraocular pressure. By taking pressure readings on a large number of patients, standards have been established for identifying both healthy and glaucomatic eyes. These standards have proven to be reasonably reliable.
The Goldmann applanation and other contact type tests require the test instrument to physically deform the cornea and therefore should only be performed by a physician or a qualified technician to avoid harming and/or stressing of the patient. It is common practice while carrying out an applanation test to anesthetize the patient's eye to minimize discomfort. This, however, poses certain risks in that harmful pressures may be developed in the anesthetized eye.
The accuracy of the applanation tests can be adversely affected by a number of variables. These include the size and shape of the patient's eye, the amount of aqueous humor escaping from the eye when the cornea is depressed, variations in the response of the sclera to the applied pressure, and uncontrolled movement of the patient's head during testing. Attempts to correct or compensate for the effects of these unwanted variables have met with limited success. Furthermore, because the applanation test requires that the instrument physically contact the cornea, potentially harmful micro-organisms can enter the patient's body through the eye fluids if the contact surfaces of the instrument are not carefully cleansed.
Lechtenstein, et al. in U.S. Pat. No. 3,545,260 discloses an eye test wherein the compliance of the cornea is used to describe intraocular pressure. In this test, the eye is immersed in a chamber containing a pressurized gas which causes the cornea to become depressed. The depth of the depressure produced by the gas at a given pressure is measured using sound waves. The waves are directed at the deformed region of the eye and the echo return is then time converted to a distance measurement from which intraocular pressure is inferred.
Lechtenstein et al. in a later U.S. Pat. No. 3,690,158 discloses another test in which acoustical energy is used to measure the impedance of the eye. Intraocular pressure is again inferred from these measurements. Here, both the eye under test and a target having a known impedance are immersed in the same liquid media and acoustical waves are directed through the media at both the eye and the target. The measured impedance of the eye is compared to that of the target to determine the health of the eye. It should be noted that a fluid tight seal must be maintained about the patient's eye throughout this test. This type of seal is very difficult to maintain. If the seal is broken or the liquid is disturbed the test results will be erroneous.